At natural gas production well sites and at other natural gas production facilities there is a requirement for process pumps to perform various applications. One application is to inject chemical into the well bore. A common example of this application is the injection of methanol into a well bore to inhibit the formation of hydrates. Another common example is the injection of a corrosion inhibitor. Still another application, specific to colder climates, is the pumping of hot glycol in order to circulate it through heat exchanger tubes contained within a process loop, thereby preventing freezing of the wellhead, water storage tanks, gas-liquid separator, flow lines and other related equipment and ancillary apparatus.
Since many natural gas production wells and their associated facilities are located in remote areas where electricity is typically unavailable, gas-driven pumps frequently are invoked to use well gas to drive pumping operations of various material flows. The well gas pressure in such common applications typically ranges from about 200 psi to about 1000 psi. Inasmuch as these gas-driven pumps require relatively low-pressure gas, typically ranging from about 30 psi to about 50 psi, in order to operate, the well gas pressure is first reduced by passing through a pressure regulator prior to being invoked to drive the pumping operations. Since this low-pressure gas cannot be returned to the high-pressure gas flow line, this low-pressure gas is exhausted to the atmosphere, thereby causing pollution and simultaneously wasting valuable gas.
An alternative and preferable approach is to use a ventless gas drive to drive process pumps. Ventless gas drives known in the art use high-pressure well gas to actuate pumping operations and then return the actuating gas to the well flow line so that no well gas is exhausted to the atmosphere. Such existing ventless gas drive designs correspond to stand-alone drive apparatus that have a reciprocating piston rod drive member that can be connected to a plurality of external reciprocating process pumps. Such existing ventless gas drive apparatus consist of a dual-acting piston within a closed cylinder with a piston rod drive member on one or both sides thereof. One or more external reciprocating pumps may then be mechanically connected to the piston rod drive member(s).
However, as is well known in the art, there are inherent problems associated with using a ventless gas drive apparatus connected to external reciprocating pumps at remote, infrequently-attended natural gas well sites where the equipment must operate continuously, i.e., operate 24 hours per day, every day of the year. It is also not unusual for natural gas wells to be located in adverse and even in harsh environments. Unfortunately, commercially-available reciprocating pumps require frequent maintenance to enable ongoing operation. In particular, such reciprocating pumps require periodic packing adjustment and concomitant lubrication-service. Reciprocating pumps are notoriously prone to both packing and seal leakage, and, therefore, in order to accommodate continual operation in the field, reciprocating pumps must be augmented with elaborate leakage-drain systems. But, ironically, while being implemented to promote continual operation of gas-driven reciprocating pumps, such elaborate leakage-drain systems, per se, require constant monitoring and maintenance.
Experience has shown that these reciprocating pumps typically need to be withdrawn from service and refurbished at least once per year. The mechanical connection between the pump and the drive unit piston rod needs frequent monitoring to assure proper alignment and free movement. Since the drive unit piston rod slides in and out of the drive unit cylinder through a sealed opening, it must be monitored closely for wear and leakage because such apparatus is sealing high-pressure well gas inside the cylinder, thereby preventing high-pressure well gas from being exhausted into the atmosphere.
Another limiting issue of ventless drive apparatus known to practitioners in the art is that the drive unit has a predefined stroke-length. This tends to limit the pumping range available from a driven external reciprocating pump which normally has a variable stroke-length. Furthermore, even though this pumping application often requires two different external pumps, e.g., an external methanol pump and an external glycol pump, and since it is not uncommon for each such external pump to have a different stroke-length and different pumping characteristics, each external pump must be directly connected to the drive apparatus piston rod having its own—usually different—stroke-length; and both external pumps must be driven at the same speed, i.e., strokes per minute, as the ventless drive. It should be evident that this situation complicates coordination of simultaneous operation of the two external pumps. For instance, for the instant exemplary field application, it is difficult for an operator to optimally set the pumping requirements simultaneously for both external pumps. This daunting challenge may compel an operator to run the external pumps at suboptimal settings.
It should be apparent that these maintenance and design issues tend to severely militate against practical application of such pumping systems at remote well sites that are only rarely scheduled to be serviced by operating personnel and that are dispersed over a wide geographical area, often in adverse and even harsh environments. As is common knowledge in the art, there are often hundreds of wells in a gas field, with frequent maintenance being neither practicable nor affordable. Moreover, in view of contemporary environmental regulations, pumping operations which are characterized by chronic seal and packing leakage problems—resulting in emissions of contaminants, including well gas, glycol, methanol, corrosion inhibitor, etc., into the environment is unacceptable and may be unlawful.
Representative of the prior art, in U.S. Pat. No. 6,694,858 and in U.S. Pat. No. 7,284,475, Grimes and Paval, respectively, disclose gas-driven reciprocating drive units, i.e., ventless gas drive units, that use a double-acting piston within a closed cylinder, in conjunction with a pressurized gas system such as a gas pipeline. A switching valve directs gas from areas of higher and lower pressure to opposite sides of the piston. The pressure differential between the two ends of the double-acting piston causes the piston to move toward a first end of the cylinder, simultaneously exhausting the gas in the first end of the cylinder back into the pressurized gas system. At or near the end of each piston stroke, the switching valve reverses the connections to the areas of higher and lower pressure in the pressurized gas system, thus inducing a pressure differential that causes the piston to move in the direction opposite to the previous stroke and thereby exhausting the gas in the second end of the cylinder back into the pressurized gas system. A piston rod connected to the piston is used to transfer the power generated by the movement of the piston to a pump or other ancillary equipment.
One of the significant drawbacks and disadvantages of the prior art exemplified by Paval and Grimes is that such embodiments of ventless gas drive apparatus are stand-alone drive units interconnected with external reciprocating pumps via a piston rod. Accordingly, embodiments of this art are susceptible to the hereinbefore elucidated operational problems and limitations. By contrast, embodiments of the present invention correspond to a pneumatic motorized multi-pump apparatus and concomitant methodology that inherently solves these problems and overcomes these limitations. As will be hereinafter described in detail, unlike the prior art, the instant pneumatic motorized multi-pump system requires neither packing nor seals that can leak pumped fluids into the environment; is devoid of a piston rod sliding in and out of a sealed opening that can leak well gas into the environment; is devoid of mechanical connections to external pumps; and requires no lubrication service.
Another significant drawback and disadvantage of the Paval and Grimes ventless gas drive units is that both prior art pump systems invoke spaced-apart circumferential piston seals to prevent flow of gas between the two ends of the drive cylinder. On the Grimes drive unit, as described by Paval, the ambient pressure within the annular space between the seals is constant and typically atmospheric at approximately 15 psi. By contrast, the gas pressure within each end of the drive cylinder may be about 1,000 psi. As a result, both of the seals in the Grimes unit are continuously working against a very large pressure differential, notwithstanding that the piston itself is exposed to only a small pressure differential. The high differential pressure acting across the seals causes high frictional forces which, in turn, reduces available power output from the drive and induces faster seal wear than if the pressure differential were significantly lower. Paval has overcome this problem via a differential shuttle valve system contained within the piston body, wherein the pressure in the annular space between the seals is always equalized to the pressure in the lower pressure end of the cylinder. It will be appreciated that this effects a differential pressure across the seals which is always equal to the pressure differential between the low pressure end of the cylinder and the high pressure end thereof. This differential pressure may be on the order of 10 to 20 psi. However, the differential shuttle valve system contained within the piston of the Paval drive introduces considerable complexity to the piston assembly which ramifies not only as higher cost, but also as more recurring maintenance.
As is well known in the industry, raw well gas often carries with it extraneous liquids such as water, condensate, etc., and extraneous solids such as sand, paraffin, pipeline debris, etc. Often the pumping system is located downstream from a gas-liquid separator apparatus and/or filter apparatus, but commonly extraneous liquids and extraneous fine solids still survive passage through these separating and filtering devices, thereby flowing throughout the pumping system. The small apertures and small passageways and moving parts of the prior art shuttle valve system are highly prone to becoming plugged and stuck under these conditions. Since the Paval shuttle valve system can be cleaned out and repaired only by taking the complete drive out of service, the Paval shuttle valve methodology therefore inflicts yet another level of maintenance concerns that militates against uninterrupted pumping operation at gas wells in the field.
As will be hereinafter described, the present invention—comprising a pneumatic motorized multi-pump apparatus and implicated systemic methodology—invokes a simpler, less costly sealing technology to maintain the differential pressure across the piston seal equal to the pressure differential between the low pressure end of the drive cylinder and the high pressure end of the drive cylinder, and it is not affected by the presence of either extraneous solids or extraneous liquids.
Yet another significant drawback and disadvantage of the Paval and Grimes ventless gas drives is the use of high pressure raw well gas to actuate the end-of-stroke switching and cycling control valves. As is well known by practitioners in the industry, raw well gas often carries with it extraneous liquids, e.g., water, condensate, etc., and extraneous solids, e.g., sand, paraffin, pipeline debris, etc. Often the pumping system will be located downstream from a gas-liquid separator and/or filter but, as is well known in the industry, some liquids and fine solids still often bypass these devices and therefore flow through the pumping system. The small apertures and small passageways and moving parts associated with the end-of-stroke switching and cycling control valves are highly prone to plugging up and sticking under these conditions. This prior art method of controlling the end-of-stroke switching and cycling of the drive unit therefore introduces still another maintenance issue to an already saturated, onerous maintenance scenario as hereinbefore described.
As will be described in detail, embodiments of the present invention, rely upon a pneumatic motorized multi-pump driver invoking a different low-maintenance methodology for controlling end-of-stroke switching and cycling of the drive unit. It will become clear that such embodiments are configured with a self-generated low pressure clean-air control circuit for actuating two low-pressure pneumatic end-of-stroke switching valves and a concomitant low-pressure pneumatic cycling valve. One of the integral pumps within the motorized multi-pump system is an air pump which supplies this low-pressure instrument air. It is a distinct advantage and feature of the present invention that this self-generated low-pressure air control circuit is completely isolated from the high-pressure raw well gas; and is inherently clean and not subject to the maintenance problems caused by intrusion of extraneous well liquids and extraneous well solids. As is well known in the industry, an additional advantage afforded by the use of a low-pressure clean-air control circuit is that low-pressure pneumatic valves, switches, and associated controls used in such a clean air instrument supply circuit have very high reliability ratings and protracted average run lives, e.g., run lives on the order of many millions of cycles.
Therefore, for the natural gas well pumping application elucidated herein, the prior art technology consists of two or more individual machines mechanically connected together, viz., a Paval-Grimes ventless gas drive unit interconnected with a plurality of driven external pumps. Such prior art pumping technology suffers from inherent operational and maintenance problems as hereinbefore described. Therefore, a need exists for a pumping apparatus and methodology that rely upon the ventless gas drive concept, but which can more adequately and more efficiently perform the demanding requirements of gas well pumping applications typically located in remote geographical venues. It will be hereinafter shown that embodiments of the present invention integrate multiple process pumps with a ventless gas drive mechanism to form a single machine satisfying the prerequisite pumping requirements of such gas well applications, thereby solving the persistent problems and overcoming the limitations that characterize such applications.